1. Field of the Invention
This invention is in the field of detectors of Manchester encoded data signals, and more particularly to a detector of predetermined patterns of values corresponding to the voltage levels of the half-bit cells of a given sequence of Manchester bit cells, some of which Manchester bit cells represent code violations.
2. Description of the Prior Art
In some token-passing local-area networks, a single communication medium is shared by many modules. Such a network is disclosed and claimed in U.S. patent application Ser. No. 540,061, filed Oct. 7, 1983, by Russell A. Henzel, and entitled "Plant Management System", which application is assigned to the same assignee as this application; namely, Honeywell Inc., the disclosure of which is incorporated herein by reference. In such a token-passing local-area network, a module which has accepted the token from another module has exclusive access to the medium to transmit information to other modules, normally for a limited period of time, at the end of which period the module having the token must transfer the token to another successor module in the network.
In such a local-area network, information is transmitted between modules in frames, where a frame is a set of binary digits. Two types of frames are used in the above-identified network. One type of frame is a token-passing frame, or token, and the second type is an information frame. In the Plant Management System referred to above, both tokens and information frames include a start-of-frame delimiter (SFD) and an end-of-frame delimiter (EFD). In that system, information transmitted by or received by the bus interface unit (BIU) of each module is in the form of binary signals which are Manchester encoded with each information bit included in a Manchester bit cell. The informational content of a Manchester bit cell is deemed a logical zero if the signal voltage level is initially low, a logical zero, in the first half-bit cell of the Manchester cell, and high, a logical one, in the second half-bit cell of the Manchester bit cell. A mid-bit low-to-high voltage transition thus occurs in the middle of a Manchester bit cell, the informational content of which is a logical zero. The informational content of a Manchester bit cell is deemed a logical one if the signal voltage level initially is high, a logical one, in the first half-bit cell; and low, a logical zero, in the second half-bit cell of the Manchester bit cell. A mid-bit high-to-low voltage transition thus occurs in the middle of a Manchester bit cell, the informational content of which is a logical one. Manchester encoding requires that there be a voltage transition, high-to-low or low-to-high, in the middle of each Manchester bit cell, a mid-bit transition. If no such mid-bit transition occurs in a Manchester bit cell, a code violation (CV) occurs.
In the Plant Management System referred to above, both start-of-frame delimiters (SFD) and end-of-frame delimiters (EFD) include code violations, four CV's each. By using CV's in such a manner, a four-bit error would have to occur to change valid Manchester encoded data into a frame delimiter.
Since every active module of the local-area network receives every information frame and token frame transmitted over the network's communication medium, it follows that the bus interface unit (BIU) of each module of the network must be able to identify in real time the unique patterns of Manchester encoded data which denote a start-of-frame and an end-of-frame delimiter since the address of the module, or modules, to which each frame is addressed immediately follows an SFD; and an EFD denotes, particularly to the module to which a token frame is addressed, that that module has the token and must transmit an information frame or transmit a token frame addressed to a successor module.
Thus there is a need to provide the BIU's of each module of a local-area network with a detector of predetermined patterns of Manchester encoded data signals substantially as received, which patterns may include code violations as well as properly encoded Manchester data.